The long-term objective of the project is to identify the cognitive mechanisms of time perception. The specific aims of this proposal are to identify the pharmacological mechanisms of dopaminergic and cholinergic modulation of time estimation and to identify the source (clock or memory) of non-linearities in timing. The rationale underlying these aims is that, without identify the source of non-linear processes in timing, it is not possible to provide a complete understanding of time perception. The central hypothesis of the proposed research is that the source of non-linearities may be identified by using behavioral and pharmacological manipulations that independently evaluate clock and memory processes. Pharmacological specificity will be established using multiple selective agonists and antagonists. To examine non-linear processes, it is necessary to examine many, closely spaced interval conditions. Non-linearities in clock and memory mechanisms will be dissociated by examining potential shifts in non-linearities under pharmacological manipulations. The overall outcome will be a precise statement of which timing component is non-linear. This result is expected to have a significant impact on the field because major theories of time perception were found on linear rather than non-linear timing. Understanding how non-linear phenomena affect time perception would be within reach for the first time. It is necessary to understand timing mechanisms because timing is involved in many aspects of behavior. Identifying the mechanisms of non-linear temporal processing is expected to have a significant impact on the field because major theories of time perception were founded on liner rather than non-linear timing.